1. Field of the Invention
This invention relates to a process for producing a polyolefin thermoplastic elastomer composition. More particularly it relates to a process for producing a partially crosslinked polyolefin thermoplastic elastomer composition suitable to extrusion molding or injection molding for thick products.
2. Description of the Related Art
In the case of subjecting conventional rubbers to injection molding, there have been problems that since the process requires steps of blending additives into a rubber, kneading the resulting blend, filling into a mold and vulcanizing, a particular molding machine is required, the cycle time becomes longer and the steps are troublesome. In the case of the extrusion molding, too, similar problems have been raised. Thus, as a stock, a composition substituted for rubbers, which is moldable even without vulcanization and yet provided with rubbery properties has been researched. Among stocks having such properties, soft plastics such as soft vinyl chloride resin, ethylene-vinyl acetate resin, low density polyethylene, etc. are superior in the aspect of both moldability and softness, but on the other hand are inferior in the aspect of heat resistance, mechanical strengths, repellent elasticity, etc.; hence these plastics have been restricted in the use applications.
Thus, in recent years, as materials having intermediate properties between those of vulcanized rubbers and those of soft plastics, compositions comprising an olefin copolymer rubber and an olefin plastic have been noted among thermoplastic elastomers. Namely, research has been made around polypropylene and ethylene-propylene copolymer rubber, for example, a composition obtained by melt-kneading a polyolefin resin with an ethylene-propylene copolymer rubber, disclosed in Japanese patent application laid-open No. Sho 49-53938/1974, a composition having an improved impact strength, obtained by blending an olefin copolymer rubber with an olefin plastic at two stages, disclosed in Japanese patent application laid-open No. Sho 60-71652/1985, etc.
Among these compositions, olefin thermoplastic elastomers having the rubbery part therein partially crosslinked at the time of melt-kneading are so superior in the heat resistance and rubbery elasticity that development thereof has been advanced as a most promising composition. Partially crosslinked olefin thermoplastic elastomers have generally been produced by mechanically melt-kneading an olefin rubber such as ethylene-propylene copolymer rubber or ethylene-propylene-non-conjugated diene terpolymer rubber with polypropylene usually by means of Bambury mixer, kneader, etc. and at the same time adding a crosslinking agent such as organic peroxide, etc. to effect partial crosslinking. As such a process for obtaining thermoplastic elastomers, for example, the following have been proposed: a process of partially crosslinking the rubbery part at melt-kneading (Japanese patent publication No. Sho 53-34210/1977); a process of crosslinking the same by using an organic peroxide, (Japanese patent application laid-open No. Sho 57-172944/1982); a process of partially crosslinking the same by using divinylbenzene as a crosslinking auxiliary (Japanese patent application laid-open No. Sho 57-135846/1982), a process of completely vulcanizing the same with sulfur (Japanese patent application laid-open No. Sho 52-13541/1977); a process of using an organosilane (Japanese patent application laid-open No. Sho 57-23651/1982); etc.
However, compositions obtained by merely melt-kneading an olefin copolymer rubber and an olefin plastic are inferior in the heat resistance and mechanical strength and also inferior in the practical utility. Further, the process accompanied with crosslinking is superior in obtaining a product having a heat resistance, a rubbery elasticity and a mechanical strength, but inferior in the processing fluidity, resulting in an inferior appearance of the resulting molded product. Further, in the case of using an organosilane as a crosslinking auxiliary, a process of post-crosslinking by immersing in hot water in the presence of a catalyst such as dibutyltin dilaurate is required; hence there occurs a new problem of inferior processability.